You could just seat the bullets another 30 thousanths in and see if the problem goes away, but for actually measureing there are a lot of ways. The Stoney point tool is one. The method that I've settled on is an adaptation of what has been called the "cleaning rod method".
The tools involved are a piece of drill-rod or other smooth rod of close to bore diameter with sqare flat ends and a couple of drill bit stop collars that will fit on the rod. Close the bolt on your empty rifle leaving it cocked. Carefully run the rod in from the muzzle until it contacts the bolt face and slide the two stop collars down until they touch the muzzle. Tighten the set screw of the top collar. Next take the rod out, remove the bolt and place a bullet in the barrel. A pencil works about as good as anything to push the bullet to the lands. Slip the rod back in until the end touches the bullet nose and push the bullet back and forth until you feel that you have positioned it against the lands with the lightest possible pressure. Set the second stop collar and measure the distance between the two collars with your dial calipers. The distance you measure is the COL to lands with that particular bullet. It may not be correct with any other bullet in the box, but it is right with that one so hang on to it and set your die with that bullet. If you seat it to that length and measure it with a comparator you will have a reference that will work with any bullet.
Now you can play a bit for the sake of science.
Try different seating pressures to see just how much the measurement changes. Simply dropping the bullet into the barrel will usually cram it 30 thousandths into the lands. If you don't believe it, try to get it to drop out under it's own weight. It will be stuck and chances are will take a tap with the rod to get out. Compare against your dented case method to find out how much difference there is.
Some patterns emerge, like a hunting bullet will usually seat 30 thou into the lands with the case method. The common practice of backing off 30 thou is usually pretty close to kissing the lands. When I think of all the HTHP Match bullets that I "Jumped" 90 to 100 thou before they would shoot, I shouldn't have been suprised to find that what I thought was touching was crammed 100 into the lands. Turns out I wasn't jumping anything. Oh well, we live and learn. And occasionally dump powder out of actions.
